While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, DUV and VUV lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using an ArF excimer laser as the light source is thought requisite to the micropatterning technique capable of achieving a feature size of 0.13 μm or less.
The ArF lithography started partial use from the fabrication of 130-nm node devices and became the main lithography since 90-nm node devices. Although lithography using F2 laser (157 nm) was initially thought promising as the next lithography for 45-nm node devices, its development was retarded by several problems. A highlight was suddenly placed on the ArF immersion lithography that introduces a liquid having a higher refractive index than air (e.g., water, ethylene glycol, glycerol) between the projection lens and the wafer, allowing the projection lens to be designed to a numerical aperture (NA) of 1.0 or higher and achieving a higher resolution. See Journal of Photopolymer Science and Technology, Vol. 17, No. 4, p 587 (2004).
In the photolithography using an ArF excimer laser (wavelength 193 nm) as the light source, a high sensitivity resist material capable of achieving a high resolution at a small dose of exposure is needed to prevent the degradation of precise and expensive optical system materials. Among several measures for providing high sensitivity resist material, the most common is to select each component which is highly transparent at the wavelength of 193 nm. For example, polymers of acrylic acid and derivatives thereof, norbornene-maleic anhydride alternating copolymers, polynorbornene, ring-opening metathesis polymers, and hydrogenated ring-opening metathesis polymers have been proposed as the base resin. This choice is effective to some extent in that the transparency of a resin alone is increased.
Studies have also been made on photoacid generators. In prior art chemically amplified resist compositions for lithography using KrF excimer laser, photoacid generators capable of generating alkane- or arene-sulfonic acids are used. However, the use of these photoacid generators in chemically amplified resist compositions for ArF lithography results in an insufficient acid strength to scissor acid labile groups on the resin, a failure of resolution, or a low sensitivity. Thus these photoacid generators are not suited for the fabrication of microelectronic devices.
For the above reason, photoacid generators capable of generating perfluoroalkanesulfonic acids having a high acid strength are generally used in ArF chemically amplified resist compositions. These photoacid generators capable of generating perfluoroalkanesulfonic acids have already been developed for use in the KrF resist compositions. For instance, JP-A 2000-122296 and U.S. Pat. No. 6,048,672 (or JP-A 11-282168) describe photoacid generators capable of generating perfluorohexanesulfonic acid, perfluorooctanesulfonic acid, perfluoro-4-ethylcyclohexanesulfonic acid, and perfluorobutanesulfonic acid. JP-A 2002-214774, US Patent Application Publication 2003-0113659 A1 (JP-A 2003-140332), and US Patent Application Publication 2002-0197558 A1 describe novel acid generators capable of generating perfluoroalkyl ether sulfonic acids.
Among these, perfluorooctanesulfonic acid and homologues thereof (collectively referred to as PFOS) are considered problematic with respect to their non-degradability and biological concentration in the environment. Manufacturers made efforts to develop partially fluorinated alkane sulfonic acids having a reduced degree of fluorine substitution as the replacement to PFOS. For instance, JP-A 2004-531749 describes the synthesis of α,α-difluoroalkanesulfonic acid salts from α,α-difluoroalkene and a sulfur compound and discloses a resist composition comprising a photoacid generator which generates such sulfonic acid upon exposure, specifically di(4-tert-butylphenyl)iodonium 1,1-difluoro-2-(1-naphthyl)-ethanesulfonate. JP-A 2004-2252 describes the development of α,α,β,β-tetrafluoroalkanesulfonic acid salts from α,α,β,β-tetrafluoro-α-iodoalkane and sulfur compound and discloses a photoacid generator capable of generating such a sulfonic acid and a resist composition comprising the same. JP-A 2002-214774 discloses such photoacid generators as difluorosulfoacetic acid alkyl esters and difluorosulfoacetic acid amides although their synthesis method is lacking. Furthermore, JP-A 2005-266766 discloses a photosensitive composition comprising a compound capable of generating a partially fluorinated alkane sulfonic acid having a sulfonylamide structure derived from perfluoroalkylene disulfonyl difluoride.
In an attempt to form a fine feature size pattern with a pitch of less than 200 nm, the problem of pattern density dependency (or optical proximity effect), that is, the size difference between isolated and grouped patterns having different optical contrast becomes significant. Using a photoacid generator capable of generating an acid with low diffusion, the problem of pattern density dependency can be overcome to some extent, but not to a satisfactory extent. While the resist composition is required to achieve a further reduction of the pattern rule as well as a good balance of sensitivity, substrate adhesion, and etching resistance, it is also required to ameliorate the pattern density dependency fundamentally without a loss of resolution.
Under the circumstances, it was proposed to form a polymer from an acryloyloxyphenyldiphenylsulfonium salt as a monomer for enhancing sensitivity (as described in JP-A 4-230645) and to incorporate the monomer into a polyhydroxystyrene resin for improving the line width roughness (LWR) of this base resin (as described in JP-A 2005-84365). However, since the sulfonium salt is bonded at its cation side to the polymer, the sulfonic acid generated therefrom upon exposure to high-energy radiation is equivalent to the sulfonic acids generated by conventional photoacid generators, which is unsatisfactory to overcome the outstanding problem. Also, sulfonium salts having an anion side incorporated into the polymer backbone such as polystyrenesulfonic acid are disclosed as effective in enhancing sensitivity or improving resist pattern profile (Japanese Patent No. 3613491). The acids generated therefrom are arenesulfonic and alkylsulfonic acid derivatives which have too low an acid strength to sever acid labile groups, especially acid labile groups in ArF chemically amplified resist compositions. JP-A 2006-178317 discloses a polymer having a plurality of partially fluorinated sulfonic acid anions as polymerizable units, and a resist material comprising the polymer. WO 2006-121096 discloses a polymer having three partially fluorinated sulfonic acid anions in combination with a specific lactone compound. JP-A 2007-197718 discloses three anions. Since they are esters of carboxylic acids which are strong acids, they are expected to be readily hydrolyzable and low stable. Copolymers derived therefrom have an insufficient solubility in resist solvents. Furthermore, JP-A 2008-133448 discloses a sulfonium salt having a partially fluorinated alkane sulfonic acid anion as a polymerizable unit, which has insufficient resist performance in terms of LWR.
With respect to the immersion lithography, some problems arise from minute water droplets which are left on the resist and wafer after the immersion exposure. They can often cause damages and defects to the resist pattern profile. The resist pattern after development can collapse or deform into a T-top profile. There exists a need for a patterning process which can form a satisfactory resist pattern after development according to the immersion lithography.
The lithography techniques which are considered promising next to the ArF lithography include electron beam (EB) lithography, F2 lithography, extreme ultraviolet (EUV) lithography, and x-ray lithography. In these techniques, exposure must be done in vacuum or reduced pressure, which allows the sulfonic acid generated during exposure to volatilize, failing to form a satisfactory pattern profile. The sulfonic acid volatilized is damaging to the exposure system. In the EB and EUV lithography, it is desired to provide a resist material capable of minimizing the influence of acid diffusion in order to comply with further pattern size reductions.